1. Field of the Invention
The present invention relates to a semiconductor crystal, and a method and apparatus of producing the same. Particularly, the present invention relates to a semiconductor crystal for use in production of a GaAs substrate and the like employed in an optical device, an electronic device, and the like, and a method and apparatus of producing such a semiconductor crystal.
2. Description of the Background Art
A GaAs crystal, for example, among the semiconductor crystals, is produced industrially by the pulling method (LEC (Liquid Encapsulated Czochralski) method), the horizontal boat method (HB (Horizontal Bridgman) method, HGF (Horizontal Gradient Freeze) method), and the vertical boat method (VB (Vertical Bridgman) method, VGF (Vertical Gradient Freeze) method). The pulling method and the vertical boat method are particularly advantageous over the horizontal boat method for producing a single crystal since the yield is improved due to the cross section of the obtained crystal being circular identical to that of the substrate and since the diameter can easily be increased due to the symmetry of the growing crystal.
As an example of an apparatus of producing a semiconductor crystal, an apparatus having a carbon heater and a crucible that stores material melt placed in a stainless steel-made high pressure chamber is known. Such an apparatus is used in the LEC, the VB, or the VGF method.
FIG. 14 shows an example of such an apparatus using a stainless steel-made high pressure chamber. A cross sectional view of a schematic structure of an apparatus of producing a semiconductor crystal employed in the pulling method is shown.
Referring to FIG. 14, the apparatus includes a crucible 2 supported by a lower shaft 4, and a carbon heater 3 in a stainless steel high pressure chamber 11. A heat insulator 5 is provided between carbon heater 3 and stainless steel high pressure chamber 11 to prevent damage of chamber 11 caused by the heat of carbon heater 3.
In growing a crystal using such an apparatus, crucible 2 is first filled with GaAs material to prepare material melt 60 by the heat from carbon heater 3. The surface of material melt 60 is encapsulated by a liquid encapsulation material 70 in order to prevent evaporation of As from material melt 60. A pull shaft 14 having a seed crystal 55 attached at the leading end is pulled upward as indicated by the arrow to effect crystal growth under a high pressure atmosphere. Thus, a GaAs single crystal 50 is obtained.
FIG. 15 shows another example of an apparatus employing a stainless steel high pressure chamber. A cross sectional view of a schematic structure of an apparatus of producing a semiconductor crystal employed in the vertical boat method such as the VB or VGF method is shown.
Referring to FIG. 15, the apparatus has seed crystal 55 placed at the lower portion of crucible 2. By moving lower shaft 4 downwards as indicated by the arrow or by shifting the temperature distribution, material melt 60 is solidified from seed crystal 55 sequentially upwards for crystal growth. The remaining structure is similar to that of the apparatus of FIG. 14. Therefore, description thereof will not be repeated.
As another example of an apparatus for crystal growth, an apparatus is known having a crucible or boat that stores material melt sealed in vacuum in a quartz ampoule, which is heated from the outer side. Such an apparatus is used in the horizontal boat method including the HB or HGF method, and in the vertical boat method including the VB or VGF method.
FIG. 16 is a sectional view showing a schematic structure of such an apparatus employing a quartz ampoule.
Referring to FIG. 16, the apparatus has crucible 2 sealed within a quartz ampoule 21. A heater 3 such as of kanthal is provided outside ampoule 21. Quartz ampoule 21 is supported by lower shaft 4. A crystal is grown by moving lower shaft 4 downwards as indicated by the arrow or by shifting the temperature distribution.
As a method of synthesizing the GaAs material for crystal growth, the As injection method of effecting reaction between the Ga in the crucible and the arsenic vapor generated by controlling the temperature of the As source outside the crucible, and the method of charging both the Ga and the As into the crucible together and raising the temperature for a direct reaction are known. Both methods are carried out in a high pressure chamber under liquid encapsulation. It is particularly noted that the latter requires high pressure of several ten atmospheres since the arsenic vapor pressure becomes considerably high.
Production of a GaAs polycrystal is carried out by cooling the material obtained as described above. Production of a single crystal is carried out by the method of charging the prepared polycrystal into a crucible as the material, and the method of growing a single crystal subsequent to the raw material synthesization.
There is the demand for a larger semiconductor crystal as the integration density of a semiconductor device becomes higher. Currently, a GaAs crystal with 4 inches in diameter is of practical usage for a compound semiconductor crystal. The need of increasing the size of such a compound semiconductor crystal has become greater to induce various research and development. However, there are many problems on the mass production of a large compound semiconductor crystal. Production of a large compound semiconductor crystal greater than 4 inches is not yet practical.
For example, when the stainless steel high pressure chamber shown in FIG. 14 or FIG. 15 is used, a heat insulating layer must be inserted between the heater and the stainless steel chamber. Accordingly, the size of the furnace becomes larger to increase the furnace cost.
According to the method shown in FIG. 14 or FIG. 15, carbon is employed for the heater material. Heating up to a temperature as high as approximately 1300xc2x0 C. is required in preparing the material melt since the melting point of GaAs is 1238xc2x0 C. Here, the vapor pressure of carbon is small even at the high temperature of approximately 1300xc2x0 C. Therefore, carbon is suitable to be used for the heater. However, carbon is an element that is electrically active in a GaAs semiconductor single crystal. Therefore, the concentration of the carbon must be controlled in order to obtain a single crystal of high quality. When the method employing the stainless steel high pressure chamber shown in FIG. 14 or FIG. 15 is to be carried out, various measures must be taken to control the electrical properties of GaAs crystal since the carbon and the synthesized GaAs reside in the same spacing. Thus, there is a problem that the furnace cost is increased.
In the case where the quartz ampoule shown in FIG. 16 is used, there was a problem that it is difficult to produce a large crystal by charging a great amount of the material since there is a possibility of deformation or breakage of the ampoule. There is also a problem that the material cannot be synthesized, in situ, since the ampoule is sealed, barring the application of the As injection method. There is also the problem that the atmospheric gas cannot be controlled after the ampoule is sealed.
Japanese Patent Laying-Open No. 7-221038 discloses an example of using silicon carbide in the annealing furnace of a semiconductor. However, this publication is silent about the advantage of using such an apparatus in the growth of a single crystal.
Japanese Patent Laying-Open No. 2-233578 discloses an apparatus of growing a semiconductor single crystal such as GaAs according to the pulling method with the entire apparatus placed in a stainless steel chamber. This apparatus is characterized in that a solid gasket is used as the heat-proof sealing material since the chamber made of silicon carbide is subjected to high temperature. However, the heat-proof sealing member has poor airtightness, so that sufficient difference between the inside pressure and the outside pressure cannot be achieved.
Japanese Patent Laying-Open No. 2-120292 discloses an embodiment that employs silicon carbide for the crucible. However, this publication is completely silent about using silicon carbide for the reactor tube. Thus, the need of a large GaAs semiconductor single crystal with at least 6 inches in diameter has become greater in response to the semiconductor device scaled to higher integration density. A semiconductor crystal of a high quality is required at low cost.
In view of the foregoing, an object of the present invention is to provide a semiconductor crystal such as GaAs that meets the requirement of high quality and large size, and a semiconductor crystal substrate using the same.
Another object of the present invention is to provide a method and apparatus of producing such a semiconductor crystal at low cost.
According to an aspect of the present invention, an apparatus of producing a semiconductor crystal is provided. The apparatus includes a reactor tube having at least one open end, said reactor tube including any one material selected from the group consisting of silicon carbide, silicon nitride, aluminum oxide, or a composite material including any one base material selected from the group consisting of silicon carbide, silicon nitride, aluminum nitride, boron nitride, aluminum oxide, magnesium oxide, mullite, and carbon as a base, and including an oxidization-proof or airtight film formed on the surface of the base; a heater arranged around and external to the reactor tube under atmospheric pressure in an ambient air atmosphere; a flange attached at the open end to seal the reactor tube; and a crucible mounted in the reactor tube adapted to receive therein a first raw material of the semiconductor crystal. The reactor tube is capable of withstanding an internal pressure of greater than 1 atmosphere while the exterior of the reactor tube is under atmospheric pressure in the ambient air atmosphere. The apparatus further includes a seal member arranged to seal a junction between the flange and the open end of the reactor tube, and a temperature maintenance system for maintaining the temperature of the seal member and the junction between the flange and the open end of the reactor tube. The seal member is an elastic member.
In the present specification, xe2x80x9cmullitexe2x80x9d refers to a mixture of aluminum oxide and silicon oxide.
As xe2x80x9coxidation-proof or airtight filmxe2x80x9d, a thin film and the like such as of silicon carbide, silicon nitride, aluminum nitride, aluminum oxide, or silicon oxide can be enumerated. Such a film can be formed by a coat on a base.
As xe2x80x9creactor tube of composite materialxe2x80x9d, a reactor tube having the above-described oxidation-proof or airtight film coat on the surface of a base formed of carbon such as graphite, or having the surface of the silicon carbide base oxidized can be used. Alternatively, a reactor tube having the oxidation-proof ability improved by forming a thin film of silicon oxide on the surface of the base by coating can be used.
In the present invention, the temperature of the elastic member is preferably maintained at not more than 400xc2x0 C., more preferably at not more than 300xc2x0 C., and further preferably at not more than 200xc2x0 C.
In the present invention, the elastic member is preferably rubber or fluorine-contained polymer.
In the present invention, the temperature maintenance system preferably includes an air-cooling arrangement around the junction.
In the present invention, the temperature maintenance system includes a heat insulator arranged between the heater and the junction. At least one heat insulator is provided. The heat insulator is preferably arranged inside the reactor tube.
In the present invention, a cooling member is preferably arranged in close proximity to and/or around the junction.
In the present invention, the cooling member is preferably a fin.
In the present invention, the cooling member according to cooled gas in the present invention is preferably arranged in close proximity to and/or around the junction.
In the present invention, the temperature maintenance system preferably includes a cooling water jacket arranged in close proximity to and/or around the junction.
In the present invention, the semiconductor crystal is preferably a GaAs crystal.
In the present invention, the semiconductor crystal also includes a silicon semiconductor, a germanium semiconductor, and the like, in addition to the compound semiconductor such as GaAs, CdTe, InAs, GaSb, and the like. Also, it is assumed that the semiconductor crystal includes a single crystal or a polycrystal.
In the present invention, the internal pressure in the reactor tube is preferably maintained to be higher than the atmospheric pressure, and the ambient pressure outside the reactor tube is preferably the atmospheric pressure.
In the present invention, the apparatus preferably includes a reservoir arranged in the reactor tube above the crucible, and a pipe that extends downward from the reservoir into the crucible. The reservoir is adapted to receive a second raw material of the semiconductor crystal. The pipe is adapted to introduce a gas of the second raw material into the first raw material in the crucible through the pipe.
In the present invention, the first raw material received in the crucible is preferably Ga, and the second raw material received in the reservoir is preferably As.
The apparatus of the present invention preferably includes a shaft member that extends through the flange at a lower end of the reactor tube and is connected to the crucible.
In the present invention, the reactor tube is preferably arranged in a vertical direction. The shaft member can move the crucible upwards and downwards in the reactor tube.
The apparatus of the present invention preferably includes at least one temperature measurement member arranged inside the reactor tube in close proximity to the crucible.
According to another aspect of the present invention, a method of producing a semiconductor crystal is provided. This method includes the steps of: a) charging the first raw material into the crucible in the reactor tube; b) attaching the flange at the open end to seal the reactor tube; c) maintaining the interior of the reactor tube under an inert gas atmosphere at an inner pressure greater than the atmospheric pressure; d) forming a material melt in the crucible by heating the reactor tube using the heater; and growing the semiconductor crystal by solidifying the material melt.
In the present invention, the step of growing the semiconductor crystal preferably employs any crystal growth method selected from the group consisting of a VB method, a VGF method and a pulling method.
In the present invention, the semiconductor crystal is preferably a GaAs crystal. The step a) includes the steps of: filling the crucible with Ga; filling a reservoir with As; and placing the crucible filled with Ga and the reservoir filled with As in the reactor tube. The step d) includes the steps of: heating the Ga in the crucible up to a temperature higher than the melting point of GaAs by the heater; heating and sublimating the As in the reservoir by the heater to produce arsenic vapor; and introducing the arsenic vapor into the Ga via a pipe from the reservoir to form a GaAs melt in the crucible.
In the present invention, the step of growing the semiconductor crystal uses any crystal growth method selected from the group consisting of a VB method, a VGF method and a pulling method.
In the present invention, the heater is preferably a resistance heater.
In the present invention, the primary component of the heater is preferably iron.
In the present invention, the heater is preferably externally unshielded and the apparatus includes no shield externally around the heater.
In the present invention, the reactor tube is capable of withstanding a temperature of at least 1250xc2x0 C.
In the present invention, the reactor tube is capable of withstanding an internal pressure of 2 atmospheres in gauge pressure when the reactor tube is at the temperature of at least 1250xc2x0 C.
In the present invention, the step of heating the GaAs in the crucible includes raising the temperature to at least 1250xc2x0 C.
The apparatus of producing a semiconductor crystal of the present invention is characterized in that a reactor tube is included formed of any one material selected from the group consisting of silicon carbide, silicon nitride, aluminum nitride, and aluminum oxide, or of a composite material with any one of a material selected from the group consisting of silicon carbide, silicon nitride, aluminum nitride, boron nitride, aluminum oxide, magnesium oxide, mullite, and carbon as the base, and having an oxidation-proof or airtight film formed on the surface of the base. Therefore, the possibility of deformation or breakage of the reactor tube such as in the case of using the conventional quartz ampoule is eliminated. A large amount of the material can be charged to produce a crystal of a large size.
The usage of a reactor tube formed of silicon carbide is advantageous in that the cost is lower than that of the conventional stainless steel high pressure chamber. Therefore, the furnace cost in producing a semiconductor crystal according to the present invention can be lowered considerably.
When a reactor tube formed of silicon nitride, formed of aluminum nitride, formed of aluminum oxide, or formed of a base that does not include carbon such as silicon nitride, aluminum nitride, boron nitride, aluminum oxide, magnesium oxide, mullite, and the like is used, introduction of carbon into the semiconductor material can be prevented. As a result, a semiconductor material of high purity can be achieved.
Even in the case where a base is used of a carbon material such as graphite that is not oxidation-proof, or a porous aluminum oxide or mullite that is not airtight, damage or fracture of the reactor tube can be prevented effectively by coating the surface of the base with oxidation-proof or airtight material. Since the material corresponding to such a base is extremely economical, a semiconductor crystal can be obtained at low cost according to the present invention.
Even in the case where a base including carbon such as silicon carbide or graphite is used, introduction of carbon into the semiconductor material can be prevented by coating the surface with a material that does not include carbon. In the case where a base of low purity such as mullite is used, a semiconductor crystal of high purity can be achieved by coating the surface with a material of high purity.
According to the present invention, the reactor tube has an open end in at least one end portion. A flange is attached at this open end. Therefore, the reactor tube can be used repeatedly, opposed to the case where the conventional quartz ampoule is used. Accordingly, the production cost can be reduced.
Since a temperature measurement member is also provided in the proximity of the crucible, crystal growth of high reproducibility can be carried out.
In contrast to the case where the ampoule is sealed, the material can be synthesized in situ. Application to the As injection method is allowed. Also, the partial pressure of the atmospheric gas within the chamber can be controlled in situ to facilitate adjustment of the impurity concentration.
According to the present invention, a semiconductor single crystal where the deviation of the carbon concentration in the direction of the length is extremely small can be obtained.
The semiconductor crystal of the present invention has a diameter of at least 6 inches. The average dislocation density is not more than 1xc3x97104 cmxe2x88x922. The defect density is extremely low.